Mr. Watt's first great improvement in the engine of Newcomen may be best understood by reference to the annexed diagram, in which _a_ represents the cylinder, and _b_ its plug or piston made to fit air-tight. The pipe _d_ is furnished with a stop-cock, by means of which the elastic vapour is occasionally admitted.--A similar pipe, furnished with a stop-cock at _f_, passes from the other side of the cylinder, and enters the vessel _g_; _e_ being the reservoir to contain water. [Illustration] If we now suppose the piston at the bottom of the cylinder, and steam admitted by the pipe _d_, its expansive force will elevate the piston, and when the air is expelled, the whole internal cavity of the tube will be filled with condensible vapour. On closing the steam-cock, and opening that connected with the vessel _g_, a portion of the vapour will immediately expand itself, and coming in contact with the cold sides of the vessel, a portion of its heat must be absorbed by the water at _e_. A new portion of steam then descends, and is also condensed, and indeed the same process continues till the whole of the steam is drawn from the tube. A vacuum being thus formed, the pressure of the atmosphere will preponderate, and the piston rod be depressed to the bottom of the tube. On closing the stop-cock _f_, a new supply of steam may be admitted by the other pipe, and after raising the piston, the process of condensation may be readily repeated. The advantages that arise from this mode of forming a vacuum are very considerable, not the least important of which, is a saving of nearly half the fuel. In the old engine, the condensing water must reduce the temperature of the internal surface of the cylinder to that of the atmosphere, before a vacuum could be produced, and when the condensing water was applied more sparingly, the elastic vapour remaining in the cylinder was found to materially reduce the pressure of the air operating above. From this it will be seen that the great advantage of Mr. Watt's apparatus consists in performing the condensation in a separate vessel, so that the cylinder is always preserved at the temperature of boiling water. Having thus produced a vacuum without the intervention of condensing water beneath the piston, Mr. Watt's next improvement consisted in closing the top of the cylinder, so that the piston-rod worked through an air-tight hole in the centre of the cap; and to ensure the necessary pressure within the cylinder, steam with an elastic force greater than that of the atmosphere was admitted above the piston. The atmospheric engine of Newcomen was thus converted into a steam-engine, and its power was easily regulated. A cylinder and piston constructed on the most improved principles may now be examined. [Illustration]